1. Technical Field
The present invention relates to the field of scatterometry metrology, and, more particularly, to aperture alignment in scatterometry metrology tools.
2. Discussion of Related Art
The varied overlay measurement technologies that rely on scatterometry require that the radiation spot be aligned within the apertures of the overlay tool. A misalignment causes a symmetry breaking in the signal which, in turn, causes an overlay error that depends on the system parameters (like wavelength, illumination numerical aperture, illumination apodization, size, shape, and apodization of the field stop), and on the target parameters (like the wafer reflectivity, the target size, and the structures in the exterior of the overlay target). This error can be sizable and may harm both the accuracy of the measurement and its repeatability. Specifically, because this error is due to a system misalignment it can be regarded as a tool-induced shift, which, at least in the overlay scatterometry case, is one of the important performance parameters that is required to be tightly bounded from above.